For most occassions a 12 week test should suffice for a definitive result.

However in cases where in the person may have an auto-immune disorder or IV drug abuse, these conditions severly damage the immune system, 24 week test maybe warrented. However 24 week test is case specific and the health care provider would be a better judge.

As far as PEP is concerned, PEP is warrented in cases, where significant exposures have occurred. Being honest, there is extreme rare data on the issue"Whether PEP affects the window period", however I doubt PEP will affect the window period of 12 weeks or HIV tests.

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